This is a great question. I read William Berstein's new
e-book last weekend, and he has some interesting things to say on the subject. At the risk of butchering a fairly complex analysis, the short version is that lump-sum investing is superior to DCA if you are dealing with a pot of money instead of a continuous stream. This is basically a market-timing argument. If you are investing a lump sum over a fixed period (say 30 or 40 years) the entry point and the sequence of returns become less important. With lump-sum investing, you are investing more dollars for more years. To quote Bernstein, "In a world with a high equity risk premium, then, lump-sum investing is usually the best choice."
He also cites a an
analysis that argues that a DCA strategy longer than one year will under-perform a simple lump sum.
However, bizlady's situation is probably neither pure lump-sum investing or DCA. If she and her DH are still adding to the portfolio, that muddies the analysis. Similarly, if she and her DH plan to start withdrawals from the portfolio in the next few years (or right now), that also complicates matters. When investing a steady stream of savings, the sequence of the returns most definitely matters. In English, buying low and selling high beats the converse every day and twice on Sunday.
So, what to do? FWIW, first, I'd read Bernstein's little e-book. At $4.50, you can't go wrong. Then, I'd probably deploy all of the cash over a 3-6 month period in a manner that reflects your over-all asset allocation plan. Try to do it any longer than that, and the market-timing gremlins will get you. 2 years later, you'll still be sitting on a bunch of (inflation eroded) cash waiting for the right time to strike.